What is an epidural steroid injection? How an epidural steroid injection work An epidural is an injection that delivers steroids directly into the epidural space in the spine. Sometimes a flushing solution (either lidocaine or normal saline) is also used to help "flush out" inflammatory proteins from around the area that may be the source of pain. The epidural space is the space between the dura mater (a membrane) and the vertebral wall and is filled with fat and small blood vessels. It is located just outside the dural sac. The dural sac surrounds the nerve roots and cerebrospinal fluid (the fluid that the nerve roots are bathed in). Epidural steroid injections for inflammation There are often inflammatory factors and other substances that generate pain (e.g. substance P) that are associated with a lumbar disc herniation (see Figure 1), and this inflammation can cause significant nerve root irritation and swelling. Steroids (corticosteroids) have been shown to reduce inflammation by inhibiting the production of substances that cause inflammation, the epidural steroid injection can be highly effective because it delivers the medication directly to the site of inflammation (see Figure 2).